In recent years, the LTE-Advanced, which is a major enhancement of the Long Term Evolution (LTE) standard, is being developed with advancement of the wireless communication technology. The LTE-Advanced is assumed to use a Heterogeneous Network (HetNet) in which a macro cell covering a large area and femtocells covering small areas coexist in the same frequency band. FIG. 8 illustrates a cell configuration of the HetNet. As illustrated in FIG. 8, the HetNet is configured such that a plurality of femtocells C12 and C13 covering small areas are included in a macro cell C11 covering a large area, the femtocells using the frequency band same as the macro cell. In the LTE-Advanced, an Orthogonal Frequency Division Multiplexing (OFDM) technology is used for downlink (DL) communication from a base station to a mobile station. When the mobile station receives a signal in a femtocell, reception quality may degrade due to interference from the macro cell.
Accordingly, a technology called enhanced Inter-Cell Interference Coordination (eICIC) is introduced to reduce inter-cell interference. The eICIC involves coordination between the macro cell and the femtocells. During part of subframes, transmission of a data signal from the macro cell is stopped so as to reduce interference. The subframe during which transmission of the data signal from macro cell is stopped is referred to as Almost Blank Subframe (ABS), which improves the reception quality of the mobile station connected to the femtocell.
However, in the eICIC, it is impossible for mobile stations other than the mobile station connected to the femtocell to receive data signals from the macro cell during the ABS. Accordingly, a technology has been introduced in the Release-11 and onward of the LTE-Advanced. According to the technology, the macro cell does not completely stop data signal transmission during the ABS but transmits a signal by using electric power lower than normal power. This technology is called Further-enhanced Inter-Cell Interference Coordination (FeICIC). The FeICIC prevents communications of the mobile stations connected to the macro cell from being interrupted by the communications performed between other mobile stations and the femtocells.
While the FeICIC reduces inter-cell interference, a Cell-specific Reference Signal (CRS) in a subframe is normally transmitted when interference control is performed by using the ABS. The CRS is a signal which identifies each base station. Placement of the CRS changes depending on the ID of the cell formed by each base station. Accordingly, interference does not generally occur between the CRSs. However, a CRS is placed at intervals of 6 sub carriers (SCs) over several symbols in a subframe. If the cells have the same reminder when each cell ID is divided by 6, the CRSs of these cells are mapped to the same position, then interference occurs. As a result, CRS reception quality degrades.
A reference signal received power (RSRP) is one of the indices indicating the reception quality. Since the RSRP is an average received signal power of the CRSs, the CRSs are used to measure the RSRP. Accordingly, the above-described degradation in CRS reception quality causes degradation in accuracy of RSRP measurement. CRS-interference cancellation (IC) is one of the technologies to improve CRS reception quality. According to the CRS-IC technology, the CRS reception quality can be improved by cancelling the CRS of interfering cells from a received signal of the mobile station so as to obtain a signal which does not include interference components. By using the CRS-IC, the degradation in RSRP measurement accuracy can also be suppressed.    Non-patent Document 1: 3GPP LTE Specifications, TS36.211-v.11.3.0.    Non-patent Document 2: 3GPP LTE Specifications, TS36.214-v.11.1.0.    Non-patent Document 3: 3GPP LTE Specifications, TS36.300-v.11.6.0.    Non-patent Document 4: “Macro-Femto Inter-Cell, Interference Mitigation for 3GPP LTE-A Downlink,” M. Huang and W. Xu, WCNC 2012 Workshop on Broadband Femtocell Technologies.
However, it is still difficult to measure the precise RSRP even by the above-described method for measuring RSRP with CRS-IC because of the following reason. That is, when measuring the RSRP, a mobile station subtracts a replica signal of interfering cells from signal components of a received signal. In this process, the mobile station sometimes subtracts part of the signal components of a measurement target cell. As a consequence, part of the RSRP that is originally included in the measurement target is excluded therefrom, so that an RSRP value smaller than an actual value (hereinafter described as “ideal value”) is calculated. As a result, the RSRP measurement accuracy may degrade.
The requirement of RSRP measurement accuracy in the 3rd Generation Partnership Project (3GPP) specifies that 90% or more of RSRP measurement values are to be included in a fixed range of the ideal value in an Additive White Gaussian Noise (AWGN) propagation environment. If there is a tendency that the RSRP is calculated smaller than the ideal value when the RSRP is measured by using the CRS-IC, it may be difficult to satisfy the requirement, which poses a problem.
In the above described RSRP measuring method using the CRS-IC, part of the RSRP that is included in the measurement target is excluded therefrom. As a result, an RSRP value lower than an actual value is reported from the mobile station to its base station. This leads to the situations where proper handover is not performed, such as the handover being untimely performed, and the handover being performed too frequently. As a result, the mobile station has a degraded reception quality and/or has an unstable connection to the base station.